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Spin-dependent transport phenomena in organic semiconductors

Bergeson, Jeremy D.
http://rave.ohiolink.edu/etdc/view?acc_num=osu1167674229

Abstract Details

2007, Doctor of Philosophy, Ohio State University, Physics.
Thin-film organic semiconductor transport can have an anomalously high sensitivity to low magnetic fields. Such a response is unexpected considering that at a modest magnetic field of 100 Oe the ratio of thermal energy to the energy associated with the intrinsic spin of charge carriers is 104 at room temperature and 102 at liquid helium temperatures. Nevertheless, we report experimental characterization of (1) spin-dependent injection, detection and transport through organic semiconductors and (2) the influence of a magnetic field on the spin dynamics of recombination-limited transport. The first focus of this work is accomplished by fabricating basic spin-valve devices consisting of two magnetic layers spatially separated by a nonmagnetic organic semiconductor. The spin-valve effect is a change in electrical resistance due to the magnetizations of the magnetic layers changing from parallel to antiparallel alignment, or vice versa. Metallic and semiconductor conductivities differ by many orders of magnitude inhibiting spin injection from the magnet into the nonmagnet, however, we achieve a 20% spin-valve effect at cryogenic temperatures by inserting ultra-thin tunnel barriers at the metal/semiconductor interfaces. In a unique device we replace one metallic magnet with the organic-based magnetic semiconductor vanadium tetracyanoethylene (V[TCNE]2) and attain a spin-valve effect at 10 K. The second focus of this work is the bulk magnetoresistance (MR) of thin-film small molecule, oligomer and polymer organic semiconductor structures. At room temperature the resistance can change significantly (up to 8% at 100 Oe, 15% at 1000 Oe.) Depending on parameters such as temperature, layer thickness, or voltage, the resistance of these materials may increase or decrease as a function of field. A model, termed magnetoresistance by the interconversion of singlets and triplets (MIST), is developed to account for this anomalous behavior. The MIST mechanism predicts that increasing the spin-orbit coupling in the organic semiconductor should decrease the magnitude of the MR. In an experiment where the small molecule Alq3 is doped with strong spin-orbit coupling molecules the MR decreases by an order of magnitude or more, depending on the doping. In addition, we show the experimental observation of high-field MR in devices with and without magnetic contacts.
Arthur Epstein (Advisor)
133 p.
Organic Semiconductor; Magnetoresistance; Space Charge Limited Current; Spintronics; Spin Injection; Spin Valve

Recommended Citations

Citations

  • Bergeson, J. D. (2007). Spin-dependent transport phenomena in organic semiconductors [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1167674229

    APA Style (7th edition)

  • Bergeson, Jeremy. Spin-dependent transport phenomena in organic semiconductors. 2007. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1167674229.

    MLA Style (8th edition)

  • Bergeson, Jeremy. "Spin-dependent transport phenomena in organic semiconductors." Doctoral dissertation, Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=osu1167674229

    Chicago Manual of Style (17th edition)

osu1167674229
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© 2006, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.